Wireless communication networks, such as the 3rd Generation (3G) and 4th Generation (4G) of wireless telephone standards and technology, are well known. Examples of such 3G and 4G standards and technology are the Universal Wireless Telecommunications System (UMTS™) and the Long Term Evolution (LTE) respectively, developed by the 3rd Generation Partnership Project (3GPP™) (www.3gpp.org).
These 3rd and 4th generations of wireless communications, have generally been developed to support macro-cell wireless phone communications, and more recently femto-cell wireless phone communications. Here the ‘phone’ may be a smart phone, or another wireless or portable communication unit that is linked wirelessly to a network through which calls etc. are connected. Henceforth all these devices will be referred to as wireless communication subscriber units. Calls may be data, video, or voice calls, or a combination of these.
Typically, wireless communication subscriber units, or User Equipment (UE) as they are often referred to in 3G parlance, communicate with a Core Network of the 3G or 4G wireless communication network. This communication is via a Radio Network Subsystem. A wireless communication network typically comprises a plurality of Radio Network Subsystems. Each Radio Network Subsystem comprises one or more cells, to which wireless communication subscriber units may attach, and thereby connect to the network. A basestation may serve a cell. Each basestation may have multiple antennas, each of which serves one sector of the cell.
Geolocation is the real-world geographical location of objects, and geolocation of wireless communication subscriber units is an increasingly important and desirable service. There are many mechanisms by which users of a wireless communication network may be located, whilst they are using the system. These include use of the global positioning system (GPS), if the wireless device is equipped with a GPS receiver and the user has enabled this on his/her device. However, many users do not enable GPS on their devices as it is typically a significant power drain on the device's battery.
Alternative known techniques for geolocating a wireless communication subscriber unit within a wireless communication network use measurement data obtained by the wireless communication subscriber unit relating to individual cells (or cell sectors) within the wireless communication network, and using such measurement data in conjunction with cell characteristic data to derive location estimates for the wireless communication. For example, such measurement data and cell characteristic data may be used to derive estimates of the distance between the wireless communication subscriber unit and the/each cell antenna. The derived distance estimate(s) may then be used along with known cell antenna locations to determine a location of the wireless communication subscriber unit.
One such technique uses signal strength measurements for cells visible to the wireless communication to determine the approximate distance of the wireless communication subscriber unit from the respective basestation for each of the cells. This distance information may then be used in conjunction with the known location for the basestations to estimate the location of the wireless communication subscriber unit.
FIG. 1 illustrates the decay of signal strength with increasing distance from a basestation site. The rate of decay varies depending upon the environment in which the basestation is placed. For example, in a city with many structures to attenuate the signal, the rate of decay of signal strength with distance will typically be relatively high, perhaps decaying at a rate proportional to l/r4, where r is the radial distance from the basestation site. Conversely, in the countryside where there are fewer structures etc. to attenuate the signal, the rate of decay of signal strength with distance will typically be lower than in a city, for example reduced to a rate proportional to l/r2.
Whilst these general values for signal strength decay rates may be used as a guide or a starting point for estimating the distance between a wireless communication subscriber unit and a basestation site, they will typically vary significantly from site to site, even within a given (e.g. urban) environment. Clearly, if this variation is known accurately for a given cell (or cell sector), at a given receive location, then it is possible to utilise signal strength alone to provide an accurate distance estimate between a wireless communication subscriber unit and a basestation site. If accurate distance estimates can be determined for a number of cell sites (e.g. a minimum of three), the intersection of signal strength contours (defined by the distance estimates) can be used to determine the likely location of the wireless communication subscriber unit. This situation is shown in FIG. 2. In practice, using a larger number of cell sites is beneficial, since this provides a greater number of sample points and minimises the impact of a gross error arising from measurements based on a particular cell-site.
In FIG. 2, three contours are shown 210, 220, 230, one for each of the three base-stations. In a perfect system, with perfectly circular contours (as shown), the intersection of the contours 210, 220, 230 at a single point would provide a precise location. In a real scenario, however, the modelled signal strength contours will be far from accurate and the measured vs modelled values will lead to a gap (as shown), within which the wireless communication subscriber unit is likely to be located. So long as this ‘gap’ is sufficiently small, a sufficiently accurate location estimate may be obtained.
Alternative known techniques for geolocating a wireless communication subscriber unit within a wireless communication network use measurement data obtained by the wireless communication subscriber unit comprising signal transmission and reception timing information. Wireless communication networks fall into two broad categories:                (i) Synchronous networks, such as Code Division Multiple Access systems, e.g. CDMA 2000. In synchronous networks, the timing offset between different basestations is constant. The amount of the offset is known to wireless mobile communication units that are using the network. In the example of CDMA2000, the timing offset is both known and constant, because each basestation's timing is locked to a Global Positioning System satellite.        (ii) Asynchronous networks, such as the Universal Mobile Telephone System (UMTS). In asynchronous networks, the timing offset between different basestations is not constant. Wireless mobile communication units in asynchronous networks are not provided with information about the timing offset between basestation timing references. In addition, these references drift over time, relative both to absolute timing references, as well as to each other.        
In a synchronous wireless communication network, the known timing offset information provides the cell characteristic data that make it relatively straight forward to derive a measurement of the location of a wireless communication subscriber unit from signal timing information. However, in an asynchronous network, it is much more difficult to derive a location estimate for a wireless communication subscriber unit from signal timing information because of the lack of cell timing offset information being available.
Furthermore, although LTE networks comprise synchronous networks, not every LTE network reports timing advance/offset information, and without this cell characteristic information the accurate judgement of distance from a cell site can be difficult.
Still further alternative techniques for geolocating a wireless communication subscriber unit within a wireless communication network are known that use alternative forms of measurement data obtained by the wireless communication subscriber unit, such as signal-to-noise ratio data, Arbitrary Strength Unit (ASU) data (for example that is proportional to the received signal strength measured by the wireless communication subscriber unit), OTDOA (Observed Time Difference Of Arrival), RSCP (received signal code power), etc.
A problem with each of these geolocation techniques that use measurement data obtained by the wireless communication subscriber unit in conjunction with cell characteristic data to derive location estimates for the wireless communication is that of deriving and keeping up-to-date cell specific characteristic data in a cost effective manner.
Thus, there is a need for a method and apparatus for enabling characteristics for cells within wireless communication networks to be accurately modelled.